There are a number of printing processes which have been in common use, both for printing continuous paper web sheets and for printing preformed articles, such as, plastic containers and lids for such containers. The printing of paper web sheets usually is effected by letterpress or relief printing, offset-lithography or gravure printing.
In letterpress or relief printing, image areas are raised in relief above non-image areas. Ink is applied by rollers to the raised surfaces and transferred directly onto paper. This type of printing uses metal type, engravings and forms combining both.
Litho plates for wet offset printing carry both the image and non-image areas on the same plane or level while those for dry offset have raised relief image areas. In wet offset, the image areas are photoprinted onto a thin metal plate that has been chemically sensitized to accept ink and repel water on the image area. The plate first contacts rollers of water or dampening solution, then inked rollers. In dry offset, the water rollers are omitted and the ink rollers contact the relief image. In both cases, the inked image is transferred or offset from the plate to a rubber blanket cylinder, and then onto paper. The resilience of the rubber blanket permits offset lithography on a wide range of texture surfaces.
Gravure printing is the opposite of relief printing, in that the etched image areas are recessed into a metal plate to form reservoirs or wells for ink. The total image area is screened with the depths of the wells controlling the amount of ink transferred and density of tone on paper. The etched cylinder rotates in a fountain of ink, so filling the wells. A doctor blade wipes ink from the non-recessed, non-image areas of the plate. The image is then transferred directly from the plate onto paper. Gravure provides quality reproduction on both smooth and textured surfaces.
When it is desired to produce multicolor printings, wherein primary printing colors, namely, yellow, red (magenta), blue (cyan) and black, overlap to provide shadings of color on the paper, separate gravure printing stations are used for each color, with the previous color image drying, as it passes from one color printing station to the next such station, before application of the next color image.
In the printing of preformed containers, offset printing generally has been used. Where multiple color printings are required, these are formed by a plurality of printing stations arranged around the blanket cylinder to print separate color images of heat set ink thereon for transfer to the container. To provide for reproducibility of the desired printing on a subsequent article and avoid quality impairment due to residual mixed paint on the blanket, the colors are printed in non-overlapping relationship on the blanket.